As a printing apparatus for cans, there is known a printing apparatus including a plate-type printer (main printer) having plural plate cylinders for printing different colors and performing printing on side surfaces of cans and an inkjet printer (secondary printer) performing additional printing on side surfaces of cans after the printing by the main printer (Patent Literature 1).
As a can inspection machine which inspects the printing state of cans, there is disclosed a can inspection machine including a can rotation device which rotates cans, a can imaging device which takes images of rotating cans and an image processing device which processes the taken images in Patent Literature 2. In the Patent Literature 2, the image processing device has an image inspection means for inspecting whether printing is correctly performed as compared with a master image, and lack of printing, stains on appearance and so on are inspected by the image inspection means. When using the can inspection machine, the unmanned operation is realized and the cost can be reduced.